Proc. Natl. Acad. Sci. USA Vol. 96, pp. 5570–5574, May 1999 Ecology

Sexually transmitted chemical defense in a ( ornatrix) (nuptial gift͞sexual selection͞pyrrolizidine alkaloids͞Lepidoptera͞Arctiidae)

ANDRE´S GONZA´LEZ,CARMEN ROSSINI,MARIA EISNER, AND THOMAS EISNER*

Section of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853

Contributed by Thomas Eisner, March 17, 1999

ABSTRACT The arctiid moth is pro- the females with protection. The tests were coupled with a tected against predation by pyrrolizidine alkaloids (PA) that quantitative assessment of the allocation of the male’s PA gift it sequesters as a larva from its food plant. Earlier work had to different parts of the female’s body and with a determina- shown that males transmit PA to the female with the sperm tion of the deterrence of PA to one of the (L. package and that the female bestows part of this gift on the ceratiola).† eggs, protecting these against predation as a result. We now show that the female herself derives protection from the gift. MATERIALS AND METHODS Females deficient in PA are vulnerable to predation from spiders (Lycosa ceratiola and Nephila clavipes). If mated with a Utetheisa. Several categories of adult Utetheisa were offered PA-laden male, the females become unacceptable as prey. The in the tests with spiders. effect takes hold promptly and endures; females are unac- Field-collected (wild) Utetheisa. These were netted as adults ceptable to spiders virtually from the moment they uncouple in and around patches of mucronata, on the grounds from the male and remain unacceptable as they age. Chemical of the Archbold Biological Station, near Lake Placid, High- data showed that the female allocates the received PA quickly lands County, FL. Such contain usaramine (1), the to all body parts. We predict that other instances will be found principal PA in C. mucronata (8). The moths were kept in of female being rendered invulnerable by receipt of individual vials with access to water (soaked cotton wad) and sexually transmitted chemicals. offered to N. clavipes within 2 days of capture. Laboratory-reared (Ϫ) Utetheisa and (ϩ) Utetheisa. For years, we have raised Utetheisa in our Cornell laboratories on The moth Utetheisa ornatrix (: Arctiidae; hence- two types of larval diet: one with and one without PA. Details forth referred to as Utetheisa) depends on pyrrolizidine alka- of the composition of these diets are given elsewhere (9). loids (PA) for defense. It sequesters the PA as a larva from its Briefly, the PA-free diet [(Ϫ) diet] is based on pinto beans, food plants (; Crotalaria spp.) and retains the chem- which are free of PA, whereas the PA-containing diet [(ϩ) icals through pupation into adulthood. Females also transmit diet] includes a supplement of seeds of Crotalaria spectabilis, PA to their eggs (1). All developmental stages of the moth are another major food plant of Utetheisa. Utetheisa raised on these protected as a result: the larvae and adults against spiders (2, diets are designated (Ϫ) Utetheisa and (ϩ) Utetheisa, respec- 3) and the eggs against ants and ladybird beetles (4, 5). tively. The (ϩ) Utetheisa had been shown to contain mono- Notably, the PA in the eggs do not stem entirely from the crotaline (2), the principal PA in C. spectabilis (3, 10). mother moth. Mating in Utetheisa involves transfer from male to female of a sperm package (spermatophore) of considerable size, amounting on average to over 10% of male body mass (6). In addition to sperm and a quantity of nutrient that the female invests in egg production (7), the spermatophore contains PA. It has been shown experimentally that in endowing the eggs, the female resorts not only to her own PA but also to a portion of the PA received from the male (4). The question of whether the female herself might benefit from receipt of the male’s alkaloidal gift remained unan- swered. Might she derive a defensive advantage from mating? Here, we provide evidence that she does and that the advan- tage takes hold virtually from the moment the female uncou- ples from her partner. Our experiments consisted of testing for the vulnerability of adult Utetheisa to predation by orb-weaving spiders (field tests Once-mated laboratory-raised females [Ϫ & (ϩ () and Ϫ & with Nephila clavipes; Araneidae) and wolf spiders (laboratory (Ϫ ()]. These females were raised on the (Ϫ) diet and were tests with Lycosa ceratiola; Lycosidae). The Utetheisa offered mated either with a (Ϫ) male and designated Ϫ & (Ϫ ()or to the spiders were of several categories, including, most witha(ϩ) male and designated Ϫ & (ϩ (). Matings were importantly, females that had been raised to be PA-free but effected by confining 2-day-old virgin males and females, in that had mated with males that were either PA-laden or PA-free. These categories permitted us to determine whether individual pairs, in small chambers (8-cm diameter and height) the alkaloidal gift from the male is itself sufficient to provide overnight. Pairs typically remain in copula for over 9 h (11).

The publication costs of this article were defrayed in part by page charge Abbreviation: PA, (s). *To whom reprint requests should be addressed. e-mail: payment. This article must therefore be hereby marked ‘‘advertisement’’ in [email protected]. accordance with 18 U.S.C. §1734 solely to indicate this fact. †This paper is no. 159 in the series ‘‘Defense Mechanisms of Arthro- PNAS is available online at www.pnas.org. pods.’’ Paper no. 158 is ref. 22.

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PA-injected virgin laboratory-raised females and their controls. Oral Sensitivity of L. ceratiola to PA. To obtain a measure These females were raised on the (Ϫ) diet and injected either of this parameter, individual female L. ceratiola were each with a PA solution (150 ␮g of monocrotaline in 5 ␮l of saline) given a live mealworm (presented in the same fashion as the or with solvent only (5 ␮l saline). Injection was effected with Utetheisa), which they promptly attacked. Once they initiated a microsyringe to one side of the posterior half of the abdomen feeding, they were stimulated by topical application of a on day 4 after pupal emergence. droplet of PA solution (100 ␮g of monocrotaline in 5 ␮lofegg Orb-Weaving Spiders (N. clavipes). Tests with this albumen), delivered with a micropipette on the mealworm as were carried out at Highlands Hammock State Park, Sebring, closely as possible to the spider’s chelicers. Control spiders Highlands County, FL, a preserved site densely shaded by live were stimulated with 5 ␮l of albumen alone. oak and cabbage palm, where N. clavipes occur naturally. The PA Analyses of Utetheisa Body Components. Samples were ␮ feeding behavior of this spider is known (12). Typically, when weighed (wet weight) and extracted for 24 h with 500 lof ͞ an flies into the spider’s web, the spider darts toward it buffer solution (2.7 g of monopotassium phosphate 2mlof ͞ and bites it with the fangs to inject venom; the spider then triethylamine 0.4 ml of trifluoroacetic acid in 4 liters of water; envelopes the insect lightly in silk and carries it to the hub of pH adjusted to 3.0 with phosphoric acid). The samples then the web to eat it. Consumption is by suctional uptake of fluid were centrifuged, and the residue was reextracted with addi- ␮ from the triturated, extraorally digested prey, a process that tional buffer solution (200 l). The two extracts were pooled, may take 1–3 h with insects the size of Utetheisa. and the mixture was analyzed by HPLC with a Hewlett– Packard 1090 Series II instrument with a diode array detector Presentation of Utetheisa to N. clavipes involved flipping ␭ ϭ ϫ ␮ individual moths from vials into webs and monitoring the ( 205; C-18 BDS Hypersil column; 250 4.6 mm; 5- l particle size). The column was eluted (1 ml͞min) with a buffer course of events. We knew from previous studies that N. ͞ ͞ clavipes reject wild Utetheisa and that they are sensitive to PA; solution acetonitrile mixture (92:8, vol vol). The PA ridelline they tend to consume mealworms (larvae of Tenebrio molitor) served as internal standard. PA Analyses of Thoracic Froth. only partially if these are treated by topical addition of When adult Utetheisa are squeezed gently or sometimes even merely touched, they emit crystalline monocrotaline (2). This spider was therefore well two bubbling masses of froth from the anterolateral margins of suited for our tests. Except where otherwise indicated, indi- the thorax (Fig. 1A). They share this propensity, which is vidual N. clavipes were given only a single Utetheisa and not obviously defensive, with a number of other arctiid reused in any test. (13). To obtain froth samples for PA analyses, individual Wolf spiders (L. ceratiola). This spider was abundant on the female Utetheisa were squeezed gently with forceps, and the grounds of the Archbold Biological Station on sandy fire lanes, ensuing froth was picked up in microcapillary tubes, extracted where we spotted them at night by using head lamps that with ethanol (100 ␮l), centrifuged, and then reextracted with reflected strongly off their eyes. buffer solution (100 ␮l). After evaporating the ethanol from For laboratory testing, spiders were taken in vials and the first extract (nitrogen stream), the two extracts were pooled transferred singly into cylindrical plastic containers (17-cm and analyzed by HPLC as above. diameter; 11-cm height; with a shallow bottom layer of sand), Statistics. All PA values are expressed as mean Ϯ SEM. to which the spiders adapted quickly. Before testing, they received water only (daily misting from a spray bottle) or, in some cases, also a mealworm 2–3 weeks beforehand. We PROCEDURES AND RESULTS presented each test item by using forceps to seize an Utetheisa Tests with N. clavipes. In tests, this spider was offered moths by a forewing and then dropping the Utetheisa from just above from five different categories. Results are shown next below the spider to directly in front of its chelcers. Utetheisa that were and in Fig. 2. accepted were pounced on by the spiders and killed by one or Wild Utetheisa (six females and seven males). With the more bites; then, the spiders slowly triturated the Utetheisa and exception of two individuals (one of each sex) that were eaten, extracted their juices, a process that usually took over 2 h. wild Utetheisa were rejected. The rejection behavior was Utetheisa that were rejected were judged to have survived the reminiscent of that noted with this spider presented with other encounter if they were alive 24 h after the attack. Spiders that inedible prey (e.g., blister beetles; ref. 14). The spider con- rejected an Utetheisa were given a mealworm within 20 min verged on the moth the moment it was dropped into the orb after the test and were tallied only if they ate this item (thus and, after briefly inspecting it with legs and palps, proceeded ruling out the possibility that the Utetheisa had been rejected to cut the moth from the web. In cutting the moth free, the because the spider was not hungry). We used only female L. spider used palps and fangs in combination to sever each of the ceratiola and tested each with only a single Utetheisa. silken strands that were entangling the moth. Liberated, the

FIG.1. (A) Female Utetheisa emitting defensive froth. (Bar ϭ 1 mm.) (B) Outcome of an encounter between a L. ceratiola spider and a PA-free Utetheisa from the Ϫ & (Ϫ () category. (Bar ϭ 1 cm.) Downloaded by guest on September 27, 2021 5572 Ecology: Gonza´lez et al. Proc. Natl. Acad. Sci. USA 96 (1999)

FIG. 2. Fate of Utetheisa moths in N. clavipes webs. Numbers above bars indicate sample sizes.

moth fell to the ground or, more often, took to the wing in midfall and flew away. FIG. 3. Fate of Utetheisa females in laboratory tests with L. Other categories of Utetheisa. Utetheisa that were rejected in ceratiola. All females were raised to be PA-free and were mated either Ϫ & ϩ ( the following categories were similarly set free: of 7 female and with PA-laden males [ ( ); solid bars] or with PA-free males ϩ [Ϫ & (Ϫ (); open bars]. Numbers above bars give sample sizes. The 6 male ( ) Utetheisa, all were rejected; of 7 female and 6 male Ϫ & ϩ ( Ϫ Ϫ & ϩ ( discrimination against the [ ( )] females is significant (G test ( ) Utetheisa, all were eaten; of 14 ( ) females, 9 were on pooled data; P Ͻ 0.0001). rejected, and 5 were eaten; and of 13 Ϫ & (Ϫ () females, all were eaten. 3-week-old females). The fate of control Ϫ & (Ϫ () females It is clear from these data that not a single moth free of PA that were accepted was drastically different. These females [i.e., from the categories (Ϫ) Utetheisa or Ϫ & (Ϫ ()] was were thoroughly macerated and typically reduced to a com- rejected by N. clavipes. The spider, however, showed at least pacted amorphous mass (except for the wings, which tended to some level of discrimination against all PA-containing Utethe- break away during the course of the meal; Fig. 1B). isa, including the females from the category Ϫ & (ϩ () whose As in the tests with N. clavipes, Utetheisa were sometimes only PA was that received at mating. There is a caveat, noted to froth when attacked, but they did not do so consis- however. The 14 data points in the Ϫ & (ϩ () category are not tently. In fact, the briefest physical contact with a PA-laden Ϫ strictly independent, because they stem from tests done with & (ϩ () female at times seemed to suffice to discourage a only six spiders. We were forced to adopt this strategy because spider; occasional rejections occurred after the spider had of a shortage of webs at the test site. However, none of the six touched no more than the wings of the moth. spiders involved were tested more than once on the same day. Fate of PA-Injected Utetheisa Females and Their Controls. A further point that held true for the encounters with all The eight females that received monocrotaline (150 ␮g) by categories of Utetheisa is that the moths sometimes emitted injection on day 4 after pupal emergence were offered indi- froth in the course of being inspected by the spider. However, vidually to L. ceratiola 5 min after injection, and all eight were emission was not invariable, and most PA-containing moths rejected by the spiders. The controls (n ϭ 10), with one that were rejected did not froth before rejection. exception, were eaten (Fisher exact test; P Ͻ 0.0005). Tests with L. ceratiola. We knew from previous work (3) that Oral Sensitivity of L. ceratiola to PA. The results were clear this spider rejects wild Utetheisa (n ϭ 20; all rejected) as well cut. All spiders (n ϭ 14) that were stimulated with the as (ϩ) Utetheisa (n ϭ 20; all rejected) but that it accepts (Ϫ) monocrotaline-containing fluid dropped the mealworm and Utetheisa (n ϭ 20; all eaten). It was not known whether a then cleansed their mouth parts by dragging them in the sand. female Utetheisa endowed with no more PA than that received The response was not always immediate, because we some- from a male was also unacceptable to the spider. To answer times failed to apply the droplet precisely between the spider’s this question, we offered L. ceratiola a total of 54 Ϫ & (ϩ () chelicers. In cases of such misapplication, the response oc- female Utetheisa that had received PA at mating and 50 Ϫ & curred later, when the spider repositioned its mouth parts and (Ϫ () females that had received no such gift. The Utetheisa came into oral contact with the fluid. Spiders stimulated with were offered to the spiders at different times after mating albumen alone (n ϭ 11) all failed to respond to application of [times ranging from day 0 (5 min to4haftertermination of the droplet (Fisher exact test; P Ͻ 0.0001). mating) to day 18]. The results (Fig. 3) show that the male’s PA Analyses. Previous work (3) had established the PA alkaloidal gift provides the female with absolute or nearly content of two of the categories of Utetheisa moths used herein. absolute protection from virtually the moment mating is Wild Utetheisa taken at the Archbold Biological Station in C. completed to day 10 after mating and even with substantial mucronata patches were shown to contain 540 Ϯ 30 ␮gof protection as late as day 18 after mating, when she is already usaramine per moth (n ϭ 31; mixed lot of males and females). senescent. The control Ϫ & (Ϫ () Utetheisa, as expected, (ϩ) Utetheisa raised on (ϩ) diet in our Cornell laboratories proved largely acceptable to the spiders. contained 630 Ϯ 50 ␮g monocrotaline per moth (n ϭ 31; mixed The three individuals from the Ϫ & (ϩ () category that lot of males and females). were scored as having been accepted by the spiders were not To obtain a measure of the amount of PA received by the in fact eaten. They were scored as accepted, because they died female at mating and of the distribution of the PA in the body within 24 h of the attack. The one individual of the 2- to 3-day of the female, we analyzed three body components separately group so tallied was injured and died from the injury; however, (abdomen, head ϩ thorax, and wings) of 24 Ϫ & (ϩ () females this individual was consumed only partly (a portion of its that had uncoupled from their partners 0–2 h beforehand. The abdomen was chewed away). The other two individuals (from results (Fig. 4) show that, even at such an early time after the day 18 group) were not injured in the attack and may simply mating, the received PA is already distributed throughout the have died of ‘‘old age’’ (spontaneous mortality is high in 2- to body of the female and present in comparable concentrations Downloaded by guest on September 27, 2021 Ecology: Gonza´lez et al. Proc. Natl. Acad. Sci. USA 96 (1999) 5573

The female Utetheisa is protected by receipt of the male’s PA, and the protection takes effect quickly. Within the first 2 h after mating, the male’s PA was already found to be evenly distributed throughout the female’s body, and froth taken from females on the day of mating already contained PA at maximal levels. All females that we offered to L. ceratiola on the day of mating, even those that were tested within minutes after the end of mating, were rejected by the spiders. We conclude that the male’s PA is distributed systemically throughout the female during mating itself, over the course of the several hours that the pair remains coupled. The fact that female Utetheisa experimentally injected with PA were rejected by L. ceratiola 5 min after injection, attests to the speed with which received PA can be deployed defensively by the moth. A single mating with a PA-laden male is evidently sufficient to convey lasting protection on the Utetheisa female. Even senescent females offered to L. ceratiola on day 18 after receipt of their alkaloidal gift proved unacceptable to the spiders. Froth from such females contained undiminished concentra- tions of PA. Over time after mating, as the female bestows PA on the eggs (4), she evidently does not exhaust her supply of the chemicals. She retains PA in sufficient quantity for her own protection, thereby exercising a strategy that appears intended to protect the egg carrier as well as the eggs. In a strict sense, our experiments with PA-free females were not realistic, inasmuch as such females are not likely to occur in nature. Female Utetheisa themselves sequester PA as larvae so that at adult emergence they already contain a self-acquired quantity of the chemicals. However, that quantity is variable, FIG. 4. Total PA content (A) and PA concentration (B) in three Ϫ & ϩ ( and it can be low, as is the case when the larvae have access body components of 24 ( ) Utetheisa females whose only PA to the leaves of Crotalaria only, rather than to the PA-rich was that received at mating. The females were dissected within 0–2 h after mating. (B) The PA is shown to be equitably distributed in the seeds as well (18). The male’s PA gift is therefore to be viewed three body components [single-factor ANOVA; P ϭ 0.32]. as an important supplement, which, during times of PA- shortage in the female, could be the bonus that ‘‘makes the in all body parts. Even the wings are already PA-laden by that difference.’’ time. From the content of the parts, we calculate the total PA Also unrealistic is the fact that our experimentally mated females were but once-mated. Utetheisa females mate on content of Ϫ & (ϩ () females to be 186 Ϯ 18 ␮g, a figure that average with 4–5 males over their life span and may take as provides a measure of the amount of PA transferred by the many as 10 or more partners (11). Over time, therefore, males at mating. Ϫ & ϩ ( females are likely to receive periodic infusions of PA from Froth samples were taken from three batches of ( ) males and accrue far larger quantities of PA than they can females at three different times after termination of mating: 0 ϭ ϭ obtain from a single mate. days (1–12 h after mating; n 20), 3 days (n 23), and 18 days An interesting feature of the mating strategy of Utetheisa is ϭ (n 11). The samples ranged in mass from 0.5 mg to 3.2 mg that the female does not accept males at random but shows (the oldest females emitted the least froth). The monocrotaline preference for those of high PA content, which are able to content for the three samples was, at day 0, 4.0 Ϯ 0.6 ␮g per bestow large alkaloidal gifts (19, 20). She is capable of dis- female (concentration ϭ 2.8 Ϯ 0.4 ␮g͞mg); at day 3, 4.1 Ϯ 0.7 cerning such males, because they produce higher titers of a ␮g per female (concentration ϭ 2.9 Ϯ 0.4 ␮g͞mg); and at day courtship . That pheromone, hydroxydanaidal, is 18, 2.7 Ϯ 0.6 ␮g per female (concentration ϭ 2.7 Ϯ 0.7 ␮g͞mg). derived chemically from PA and therefore is produced in The froth is evidently laden with PA shortly after mating, at a larger quantities by PA-rich males (20). In exercising mate concentration that does not attenuate over time (single-factor choice, therefore, the female accesses those males best able to ANOVA; P ϭ 0.95). meet her defensive needs (1). Orb-weavers and lycosids are not likely to be the only spider enemies of adult Utetheisa. Crab spiders (Thomisidae) and DISCUSSION jumping spiders (Salticidae) could be natural predators as well The notion of transmission of defensive chemicals by seminal and could also be deterred by PA. The PA could also protect infusion from a male to female insect is not new. We ourselves Utetheisa against entirely different enemies, including possibly had shown such chemical transfer to occur in a danaid but- vertebrates, but evidence to this effect is lacking. terfly, Danaus gilippus, where the male, as in Utetheisa, trans- The defensive strategy exemplified by Utetheisa, in which the mits PA that it sequesters from plants to the female (15), and males help to protect not only the eggs but also the female partner, seems to be without parallel in nature. We predict, in a pyrochroid beetle, Neopyrochroa flabellata, where the however, that the strategy will be shown to have counterparts terpenoid toxin cantharidin is bestowed on the female (16, 17). in insects, where males so often transmit sizeable spermato- Utetheisa In all these cases, including , the assumption had been phores to females (21). One can certainly imagine the strategy that the beneficiaries of the transfer are the offspring, and being at play in such insects as D. gilipus and N. flabellata, evidence did indeed indicate that the eggs of Utetheisa and N. where seminal transfer of defensive chemicals has been estab- flabellata derive protection from the PA or cantharidin they lished (15–17). receive from their respective fathers (5, 17). What we have established now with Utetheisa is that the female herself profits We thank Ken Amerman for excellent laboratory help, the staff of from receipt of the male’s chemical gift. the Archbold Biological Station for numerous favors, and Jerrold Downloaded by guest on September 27, 2021 5574 Ecology: Gonza´lez et al. Proc. Natl. Acad. Sci. USA 96 (1999)

Meinwald and W. Mitchell Masters for comments on the manuscript. 11. LaMunyon, C. W. & Eisner, T. (1993) Proc. Natl. Acad. Sci. USA This study was supported in part by National Institutes of Health Grant 90, 4689–4692. AI02908 (to T.E.) and by Johnson & Johnson Fellowships in Chemical 12. Robinson, M. H. & Mirick, H. (1971) Psyche 78, 123–139. Ecology (to C.R. and A.G.). 13. Dethier, V. G. (1939) J. N. Y. Entomol. Soc. 47, 131–144. 14. Smedley, S. R., Blankespoor, C. L., Yang, Y., Carrel, J. E. & 1. Eisner, T. & Meinwald, J. (1995) Proc. Natl. Acad. Sci. USA 92, Eisner, T. (1996) Zoology 99, 211–217. 50–55. 15. Dussourd, D. E., Harvis, C. A., Meinwald, J. & Eisner, T. (1989) 2. Eisner, T. & Meinwald, J. (1987) in Pheromone Biochemistry, eds. Experientia 45, 896–898. Prestwich, G. D. & Blomquist, G. J. (Academic, Orlando, FL), 16. Eisner, T., Smedley, S. R., Young, D. K., Eisner, M., Roach, B. pp. 251–269. & Meinwald, J. (1996) Proc. Natl. Acad. Sci. USA 93, 6494–6498. 3. Eisner, T. & Eisner, M. (1991) Psyche 98, 111–118. 17. Eisner, T., Smedley, S. R., Young, D. K., Eisner, M., Roach, B. 4. Dussourd, D. E., Ubik, K., Harvis, C., Resch, J., Meinwald, J. & & Meinwald, J. (1996) Proc. Natl. Acad. Sci. USA 93, 6499–6503. Eisner, T. (1988) Proc. Natl. Acad. Sci. USA 85, 5992–5996. 18. Conner, W. E., Roach, B., Benedict, E., Meinwald, J. & Eisner, 5. Hare, J. F. & Eisner, T. (1993) Oecologia 96, 9–18. T. (1990) J. Chem. Ecol. 16, 543–552. 6. LaMunyon, C. W. & Eisner, T. (1994) Proc. Natl. Acad. Sci. USA 19. Conner, W. E., Eisner, T., Vander Meer, R. K., Guerrero, A., 91, 7081–7084. Ghiringelli, D. & Meinwald, J. (1980) Behav. Ecol. Sociobiol. 7, 7. LaMunyon, C. W. (1997) Ecol. Entomol. 22, 69–73. 55–63. 8. Sauthon, I. W. & Buckingham, J. (1989) Dictionary of Alkaloids 20. Dussourd, D. E., Harvis, C. A., Meinwald, J. & Eisner, T. (1991) (Chapman & Hall, London). Proc. Natl. Acad. Sci. USA 88, 9224–9227. 9. Bogner, F. & Eisner, T. (1991) J. Chem. Ecol. 17, 2063–2075. 21. Mann, T. (1984) Spermatophores (Springer, Berlin). 10. Johnson, A. E., Molyneux, R. J. & Merrill, G. B. (1985) J. Agric. 22. Knight, M., Glor, R., Smedley, S. R., Gonza´lez,A., Adler, K. & Food Chem. 33, 50–55. Eisner, T. (1999) J. Chem. Ecol., in press. Downloaded by guest on September 27, 2021